1. Field of the Invention
The present invention relates generally to hard copy methods and apparatus, more particularly to print media handling and, more specifically to a print media transport belt used in a wet colorant printing apparatus such as an ink-jet printer.
2. Description of Related Art
For convenience of explanation, while the present invention is useful for all wet colorant hard copy apparatus, it is described with respect to an ink-jet printer. The use of this exemplary embodiment is not intended as a limitation on the scope of the invention nor should any such intention be implied as the invention can be adapted to implementations using other wet colorant printing techniques, e.g., pigment-based and dye-based inks used in ink-jet printing, wet toner laser printing systems, and the like, also synonymously referred to hereinafter as a "wet dye" printing; the terms "colorant" or "dye" shall be construed to encompass both color and black and grey scale wet printing techniques.
The art of the exemplary embodiment in ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the assignee's Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No.1 (February 1994) editions, incorporated herein by reference for general background. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy [sic] Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988). Further details to facilitate an understanding of the present invention are provided below with respect to FIG. 1. For convenience of description, print media of all shapes, sizes, and varieties are referred to hereinafter simply as "paper."
A well-known phenomenon of wet-colorant printing is "paper cockle," the irregular surface produced in paper by the saturation and drying of ink deposits on the fibrous medium. As a sheet of paper gets saturated with ink, the paper grows and buckles in a seemingly random manner. Paper printed with images are more saturated with colorant than simple text pages and thus exhibit great paper cockle. Colors formed by mixing combinations of other color ink drops form greater localized saturation areas and also exhibit greater cockle tendencies.
In general, the prior art has approached the problem by using ribs that support the sheet of paper at various points along the width. In the state of the art, ink-jet printhead to paper separation is only about 0.5 to 1.5 millimeters. These ribbed paper support constructs allow any cockle growth to grow down away from the printing mechanism instead of toward it where contact could occur and cause further problems. Moreover, the use of ribbed paper support constructs alone is not adequate for larger dimensioned print zones. In order to hold print media flat under larger printheads a different holddown is required, such as a vacuum platen.
It has been discovered that media fiber directionality has a significant influence on the directionality of cockle growth. Papers generally have a grain in one direction longer than the other. Rib support constructs may thus be inadequate, depending on the nature of the media being printed. Feeding a sheet into a printing station of the hard copy apparatus with an orientation that minimizes cockle is impractical since it would limit the end-user's ability to select different media types.
There is a need for a paper transport and printing station paper holding device to substantially reduce and for practical purposes substantially eliminate problematical paper cockle.